1. Field of the Invention
The present invention relates to a process for manufacturing cellulose acetate, which is useful as a biodegradable plastic and is made from corncob meal.
2. Description of the Related Art
Biodegradable plastic is a plastic, which, like any ordinary plastic, exhibits excellent functions when in use, but which is quickly decomposed by microorganisms in a natural environment (for example, in the soil) after use and eventually becomes organic components of earth, water and carbon dioxide, and is drawing attention in connection with the current problem of waste, etc.
Various kinds of biodegradable plastic products have been publicized. Examples of such products include polylactic acids produced by dehydration and polymerization from lactic acid obtained by fermenting starch of corn, potatoes, etc. with lactobacilli. Such products are used for an agricultural multi-film, a compost bag, etc. However, prices of raw materials and processing costs for products are high, and these products are not necessarily rational in consideration of foodstuff situations in the future. Polycaprolactone, which is given as another example of a biodegradable plastic, is also so expensive that it is difficult to use polycaplolactone as an agricultural material, etc., and use is limited to medical materials, etc., although polycaplolactone may be satisfactory in physical properties as a plastic and biodegradability.
Moreover, a plastic obtained merely by kneading corn starch with polyethylene is being sold as a biodegradable plastic. This plastic, however, is not a biodegradable plastic in the true sense of the word, since it has become clear that, although its constituent, which is derived from natural matter, such as starch, may be biodegradable, polyethylene does not undergo any change (decomposition). Such a product is being driven out of the market despite its low price.
Thus, spreading of the biodegradable plastics, which have been heretofore known, has been slow because of their unsatisfactory performance, or because they require a complicated process for manufacture and their prices are high. The demand for biodegradable plastic products is, however, expected to increase more and more in the future for protection of the global environment, and accordingly, there is a desire for the development of products having higher performance and lower costs. Under these circumstances, studies are being performed for a biodegradable plastic composed mainly of cellulose, which plants contain in a large quantity, or a derivative thereof. However, a high cost of manufacture of this biodegradable plastic is a problem, as is the case with other biodegradable plastics.
On the other hand, the majority of a corncob is composed of cellulose (lignocellulose and hemicellulose). Corncob meal, which is obtained by drying and crushing corncobs, is used as a fungal bed for growing mushrooms, an abrasive for pulse, a nest building material for animals, etc., but very little as an industrial material. The greater part of the corncobs produced is thrown away as waste. Incineration is a main method for waste disposal, thus, there are a lot of problems with waste disposal including degradation of the environment. Study is, therefore, under way for the effective use of corncobs.
When corncobs are used as a raw material for manufacturing a biodegradable plastic consisting mainly of cellulose or a derivative thereof, etc., the cost of the raw material is zero, as hardly any labor is required for gathering the raw material, etc., and costs that have hitherto been borne by agricultural producers for waste disposal are no longer incurred. Accordingly, a biodegradable plastic made from corncobs is considered to be highly price-competitive, compared to other biodegradable plastics.
However, despite having the features mentioned, there has not been developed any biodegradable plastic consisting mainly of cellulose or a derivative thereof, etc. made from corncobs. A possible reason for this is a high cost of esterification, etc., since it is difficult to obtain cellulose (pulp of high quality) by separating lignin from lignocelluloses of which corncobs mainly consist. The separation of lignin from lignocelluloses requires a lot of steps, i.e. grinding corncobs in a stone mill, boiling with alkali and applying a sulfurous acid treatment.